In the context of the invention, "formose" is understood to mean the known mixtures of low molecular weight polyhydroxyl compounds (polyhydric alcohols, hydroxy aldehydes and hydroxy ketones) which are formed in the condensation reaction of formaldehyde hydrate.
The production of mixtures of polyhydric alcohols, hydroxy aldehydes and hydroxy ketones from formaldehyde hydrate is known and is described in numerous literature references. In this connection, reference is made, for example, to Butlerow and Loew, Annalen 120, 295 (1861) and J. pr. Chem. 33, 321 (1886); Pfeil Chemische Berichte 84, 229 (1951); Pfeil and Schroth, Chemische Berichte 85, 303 (1952); R. D. Partridge and A. H. Weiss, Carbohydrate Research 24, 29-44 (1972); Emil Fischer's Formoses of Glyceric Aldehyde and Dioxy Acetone; German Pat. Nos. 822,385, 830,951 and 884,794; U.S. Pat. Nos. 2,224,910, 2,269,935 and 2,272,378 and British Pat. No. 513,708.
However, despite the presence of all the prior art noted above no commercially workable process has yet been developed for synthesizing polyhydroxyl compounds by the autocondensation of formaldehyde. This is because the known processes are attended by certain disadvantages such as poor volume-time yields; formation of colored secondary products; inadequate reproductibility of the hydroxyl functionality of the formoses; and the elaborate operations required for removing the bases used as auxiliary reagents. As a result, the synthesis of polyhydroxyl compounds by the autocondensation of formaldehyde hydrate has appeared to be uneconomical and has prevented the autocondensation of formaldehyde hydrate from being used as a basis for a commercial process, for example, for the synthesis of polyhydric alcohols. Due to the simultaneous disproportionation of the formaldehyde into methanol and formic acid, the yields obtained with conventional processes have generally only been moderate, with the result that working up of the aqueous or aqueous/alcoholic formose solutions formed has involved considerable costs.
It is known that the disproportionation of formaldehyde into methanol and formic acid is catalyzed to a large extent by basic compounds. As Pfeil, in Chemische Berichte 84, 229 (1951) observed, the reaction velocity of this so-called "Cannizzaro reaction" is dependent upon the square of the formaldehyde concentration, whereas in the polyaddition of formaldehyde (C-C-linkage) the reaction velocity is linearly dependent upon the formaldehyde concentration (Pfeil and Schroth, Chemische Berichte 85, 303 (1952)). With increasing aldehyde concentration, therefore, the quantitative ratio of the desired polyhydroxyl compounds to methanol and formic acid is displaced against the required compounds. Accordingly, in numerous conventional aldehydes and hydroxy ketones is carried out in solutions having low formaldehyde concentrations in order to keep the quantity of secondary products as small as possible. In order to recover the hydroxy aldehydes and hydroxy ketones formed, however, the water used as solvent has to be removed again by distillation. This involves considerable energy costs because of the intense heat of evaporation of the water. For this reason, processes for the condensation of formaldehyde in dilute aqueous solutions are uneconomical. In addition, decomposition and discoloration reactions involving the hydroxy aldehydes and hydroxy ketones formed generally occur with prolonged distillation times. Accordingly, it is desirable to be able to carry out the condensation of formaldehyde in formalin solutions of standard commercial concentration in the absence of troublesome secondary reactions.
In order to avoid the Cannizzaro reaction, it has also been proposed to carry out the condensation of formaldehyde in solutions in the presence of methanol, ethanol or other polar organic solvents. The addition of organic solvents, however, again reduces the formaldehyde content of the solution. Accordingly, the additional energy costs involved in evaporating the solvent added during the working up of the hydroxy aldehydes and ketones formed also make these processes appear uneconomical. In addition, unstable semiacetals are formed from formaldehyde and lower alcohols. These semiacetals decompose during the condensation reaction with spontaneous liberation of the alcohols. For this reason, considerable delays in boiling occur during condensation reactions carried out at temperatures above the boiling point of the particular alcohol used, particularly in the case of relatively large batches. As a result, the condensation processes cannot be carried out safely on a large scale.
Accordingly, the object of the present invention is to provide a technically simple process by which it is possible to synthesize mixtures of polyhydroxyl compounds substantially free of secondary and decomposition products in favorable volume-time yields. The auxiliary reagents used (catalysts, bases) should easily separable from the reaction products. The mixtures of polyhydroxyl compounds obtained should be colorless and should require no further purification.
Another object of the present invention is to control the autocondensation of formaldehyde in such a way that the product distribution of the mixtures of low molecular weight polyhydroxyl compounds formed may be varied as required and may be reproducibly adjusted.
The solutions to these problems, however, presented difficulties for the following reasons. The normal lead-catalyzed synthesis of formose only takes place if the pH-value of the reaction mixture is adjusted to certain values with additional bases (cf. British Pat. No. 513,708). The alkali hydroxides preferably used for this purpose, however, can only be removed from the reaction product with considerable expense, for example, by using ion exchangers. In general, the tertiary amines often used, even when they are readily volatile as in the case of trimethylamine, can only be quantitatively separated from the formose with ion exchangers (the salts of the amines formed during the reaction cannot be removed from the formose by distillation). The use of ion exchangers for completely desalting the formose is, however, uneconomical, due to the large quantities of waste water involved. One possible answer to these difficulties would be to use basically reacting metal compounds as catalysts, because in this way the quantity of the foreign ions introduced into the formose would remain small.
The calcium hydroxide described by O. LOEW (J. prakt. Chem. 33, 321 (1886) as catalyst base for the production of formose from 4% aqueous formaldehyde would be eminently suitable both for ecological and for economic reasons. Ca(OH).sub.2 catalyzes the formose reaction, simultaneously regulates the pH value of the reaction mixture, and can readily be separated from the reaction product as a non-toxic compound, for example by a precipitation reaction with sulphuric acid. According to E. PFEIL (Chem. Berichte 84, 229 (1951) however, Ca(OH).sub.2 is an extremely effective catalyst for the Cannizzaro reaction. Thus, it is necessary either to accept secondary reactions, which result in reduced yields in the formation of formose, or to use highly dilute formaldehyde solutions, which is also unfavorable for economic reasons.
According to E. PFEIL and to German Pat. No. 822,385, thallium hydroxide is said to give considerably better results than Ca(OH).sub.2 in the synthesis of formose because it selectively catalyzes formose formation at the expense of the Cannizzaro reaction. However, the yields of this process are also relatively low, i.e. from 70 to 80%. Additionally, the extremely high toxicity of the thallium compounds is a deterrent to their commercial use.
The third catalyst base which is known for the synthesis of formose is Pb(OH).sub.2 or PbO. According to German Pat. No. 564,678, a mixture of C.sub.2 -, C.sub.3 - and C.sub.4 -carbohydrates is synthesized from a 4% aqueous formaldehyde solution with the addition of 125 g of Pb(OH).sub.2 per kg of HCHO and is subsequently hydrogenated to form the polyalcohols (65% yield). In addition to a high consumption of energy, however, the working up of reaction mixtures as dilute as these also involves considerable technical outlay.
It is necessary according to U.S. Pat. No. 2,224,910, to add not only from 100 to 150 g of PbO per kg of HCHO, but also to add from 1 to 3% by weight (based on HCHO), of compounds capable of enediol formation as co-catalyst at the beginning of the reaction in the synthesis of formose from 10 to 25% aqueous formaldehyde solutions (74-84% yield). According to this literature reference, the most effective quantity of co-catalyst is from 1 to 10% by weight (based on anhydrous formaldehyde) and any increase in the proportion of enediol formers beyond 10% does not afford any further advantage.
Accordingly, it must be regarded as all the more surprising that, as has now been found, the use of more than 15% by weight (preferably more than 20% by weight and, most preferably more than 40% by weight) of co-catalyst (enediol former) is of particular advantage for converting concentrated aqueous formaldehyde solutions (more than 25% by weight and preferably from 30 to 70% by weight of HCHO) into formose.
In this case, preferably only from about 20 to 80 g and, most preferably, from 30 to 60 g of PbO are required per kg of HCHO. It has also surprisingly been found that corresponding molar quantities of other lead (II) compounds, which have considerably weaker basic properties than PbO, also have good catalytic and adequately basic properties under the conditions of the process. It is particularly surprising, however, that in the process according to the present invention (by comparison with the prior art), it is only necessary to use extremely small base equivalents in the form of basic lead (II) compounds.